1. Field of the Invention
This invention relates to a method of operation of an air cleaner and particularly to a method of operation of an air cleaner used in a motor vehicle.
2. Description of the Prior Art
An air cleaner which can be used for a motor vehicle comprises an air filter, a blower motor, a gas sensor, and a control circuit for controlling rotation of the blower motor in response to an output of the gas sensor. FIG. 10 is a block diagram of such an air cleaner known in the prior art. In FIG. 10, the control circuit 13 controls the blower motor 15 through a drive transistor 14 in accordance to a sensor output V.sub.G of the gas sensor 12. Only when the engine of the motor vehicle is operating is the gas sensor 12 turned on. This prevents power consumption from the battery because generally, the gas sensor 12 contains a heater requiring large power consumption. The heater of the gas sensor 12 is provided to maintain a temperature of a ceramic catalyst at around 200.degree. C. for detecting a reducing gas through ceramic reducing catalyst reaction.
The gas sensor 12 shows an initial turn-on characteristic, as shown FIG. 11. In FIG. 11, at first, a sensor output V.sub.G increases from turn-on time t.sub.O. Then, the sensor output V.sub.G decreases via maximum V.sub.GP and reaches to a substantial convergent voltage V.sub.GO. An interval T is necessary for convergence after turn-on of the gas sensor 12.
The control circuit 13 produces a drive signal in accordance with change of the sensor output V.sub.G with respect to the convergent voltage V.sub.GO as a reference. In other words, the convergent voltage V.sub.GO is also used for a reference voltage V.sub.GO. This reference voltage V.sub.GO cannot be determined previously because it varies with characteristic deviation, temperature, humidity, and concentration of ambient gases. Thus, it is better that the convergent voltage V.sub.GO is determined when the gas sensor 12 is operated. Moreover, detection of concentration of a gas is difficult during the interval T. If concentration of a gas increases during the interval T, the detected sensor output V.sub.G increases. Thus, air pollution cannot be detected and air cleaning is not performed unless the concentration of the gas becomes higher than this detected output V.sub.G as the reference V.sub.GO. Therefore, there is a drawback that the reference cannot be determined if concentration of gas to be detected is not a normal value during the interval T.